802.1s - Multiple Spanning Trees

IEEE 802 local area networks (LANs) of all types may be connected
together with media access control (MAC) bridges. IEEE Std 802.1Q
specifies the operation of virtual local area network (VLAN) bridges,
which support VLAN operation within an IEEE 802 bridged LAN. This
Supplement to IEEE Std 802.1Q adds the facility for VLAN bridges to
use multiple spanning trees, providing for traffic belonging to
different VLANs to flow over potentially different paths within the
virtual bridged LAN.